I've just come back from flying US Airways to Miami via Charlotte on the way out and Pittsburgh on the way back, but I've flown via Philly before.
I must admit it wasn't my best journey with them, I have flown US Airways a lot, there was a delay on the way out (due to technical difficulties) that meant I missed my connection. They were very efficient at rebooking the connecting flight and it was all done before I touched down in Charlotte, just leaving me to get a different boarding card at the transfer desk. It's hard to be too critical because all airlines get these problems from time to time but one of the other passangers who claimed to be a regular ( monthly) with them said that financial constraints ( again all the airlines are facing these problems) has meant they are not as quick to repair non essential stuff ( TV monitors, armrests, tray tables etc) as they have previously been. Looking about the condition of the planes that did seem to be a justified complaint, but I'm only making that call from the 4 flights I made this trip ( 2 of the 4 planes I noticed those type of "problems"). I would also say that the food has probably slipped a notch from a couple of years back when I thought they were among the best, I would now say they are very much in the middle of the pack. I would put them on a par with United and Continental now as far as service/food quality ( a touch behind BA and Virgin), but their seats are bigger than both the British companies if that is an issue. Philly is a pretty good airport to transit in and I've always been happy to go in and out of there.
Missing a connection is always a risk when taking a non direct flight, but I couldn't fault their handling of that situation. Without doubt US Airways have slipped a little over the last couple of years, if they are flying Airbusses on the Philly route ( I believe they do) that is their newest planes and it should mean a much lower chance of any of the minor issues I mentioned being a factor. If the cost is less than £50 pounds per passenger difference , I'd pick the direct flight. I didn't follow that advice because I'm a "frequent flyer" on USAirways and they are the only company that flies to Pittsburgh ( home of DG Jana) so the airmiles are worth the extra agg to me. Mid Sept you run a high risk of weather upsetting your return flights, on a direct flight you may have a delay, but you don't run the risk of missing the transatlantic connection and having to spend a night in Philly.